There is a problem where some anti-virus vendors are using techniques to bypass Kernel Patch Protection by injecting a hypervisor which they use to intercept syscalls and make assumptions about memory locations — memory locations which are now changing with the Meltdown fixes.

To be honest, some of the techniques are similar to ones used by rootkits — Kernel Patch Protection was introduced by Microsoft a decade ago to combat rootkits, in fact. Because some anti-virus vendors are using very questionable techniques they end up cause systems to ‘blue screen of death’ — aka get into reboot loops. This shouldn’t be possible in the latest operating systems, but some anti-virus vendors have managed it by taking themselves into the hypervisor — or “hardware assisted” as you’ll sometimes read in marketing material. Anti-Virus makers really shouldn’t be messing with systems like this.